Mobile communication systems enable a mobile terminal (a.k.a., user equipment (UE)) to access a network via a network node (e.g., a base station). In some systems, before the UE begins transmitting traffic to the network via the network node, the UE performs a random access (RA) procedure to request access to the network. For example, the UE transmits an access burst to the network node using a random access channel.
To distinguish between different UEs performing RA, the access burst transmitted by the UE contains a preamble randomly chosen by the UE that the network node may use to identify the UE. Generally, the UE will uniformly, randomly select a preamble from a set of preambles (e.g., 64 preambles) that was derived from one or more root sequences (e.g., Zadoff-Chu sequences) associated with the network node.
A set of one or more preambles may be derived from a root sequence by cyclic shifting of the root sequence. The number of preambles that can be derived from a root sequence depends on the maximum expected round trip time between the UE and the network node. For instance, if a root sequence had a length of 800 μs and a very short maximum expected round trip time, then 64 preambles could be derived from the root sequence if the cyclic shift length was less than or equal to 800/64 μs, or 12.5 μs. The maximum expected round trip time is not always short. Indeed, sometimes it may be rather large. Because the cyclic shift length must be large enough to avoid any ambiguity in preamble detection due to propagation round trip time, in some instances, multiple root sequences (e.g., 64 root sequences) are required to generate 64 unique preambles.
According to some standards, a number of root sequences are available to derive the preambles. For instance, according to the 3G Long-Term Evolution (LTE) standard, a total of 838 root sequences are available for use. Each network node in the network is typically associated with a subset of the 838 root sequences. While different nodes may have the same root sequences associated with them, it is generally advisable to assign different root sequences to nodes that are physically near each other to avoid ambiguity.
Not all root sequences have the same properties. For instance, different root sequences can have different power back-off metrics (PBM) (e.g., different cubic metric (CM), peak-to-average power ratio, out-of-band emissions, etc.). All preambles derived from a particular root sequence inherit the PBM properties of the particular root sequence. While it may be desirable for a UE to select a preamble having the “best” PBM characteristics when randomly accessing a network node, all UEs should not use the preamble with the best PBM characteristics because this would result in an increase in collisions. Thus, there exists a need in the art for a method of selecting a preamble with desirable PBM characteristics while at the same time not exacerbating the collision problem.